I. Field
The present invention relates generally to telecommunications and shared frequency broadcasting, such as used in terrestrial digital multimedia/television broadcasting systems.
II. Background
Typical broadcast distribution systems include terrestrial, satellite, cable, microwave and other transmission, for data broadcasting, Internet and other wideband multimedia information transmission, and for integrated data service applications.
Terrestrial broadcasts have an advantage of strong signals within a localized area. A disadvantage is that terrestrial broadcasts have substantial signal attenuation as a result of line-of-sight limitations. Satellite broadcasts, on the other hand, provide good area coverage, but have limited power. In many cases, overhead obstructions, such as buildings, trees, etc. limit satellite reception. In the case of reception under varying conditions, such as by a mobile wireless communication device (WCD), either satellite or terrestrial broadcasts can provide the best coverage, depending on the particular circumstances at any given time.
A wireless communication device (WCD) includes but is not limited to a user equipment, station (STA), mobile station, fixed or mobile subscriber unit, pager, or any other type of device capable of operating in a wireless environment. When referred to hereafter, an access point (AP) includes but is not limited to a base station, Node-B, site controller, WLAN access point or any other type of interfacing device in a wireless environment.
OFDM
OFDM (orthogonal frequency division multiplexing) is a multi-carrier modulation technique that effectively partitions the overall system bandwidth into multiple (N) orthogonal frequency subbands. These subbands are also referred to as tones, sub-carriers, bins, and frequency channels. With OFDM, each subband is associated with a respective sub-carrier that may be modulated with data.
In an OFDM system, a transmitter processes data to obtain modulation symbols, and further performs OFDM modulation on the modulation symbols to generate OFDM symbols, as described below. The transmitter then conditions and transmits the OFDM symbols via a communication channel. The OFDM system may use a transmission structure whereby data is transmitted in symbols or groups of symbols, with each symbol transmission having a particular time duration. The symbol transmission generally includes a cyclic prefix. The receiver typically needs to obtain accurate symbol timing in order to properly recover the data sent by the transmitter. For example, the receiver may need to know the timing of each symbol transmission in order to properly recover the data sent in the symbol transmission. The receiver often does not know the time at which each OFDM symbol is sent by the transmitter nor the propagation delay introduced by the communication channel. The receiver would then need to ascertain the timing of each OFDM symbol received via the communication channel in order to properly perform the complementary OFDM demodulation on the received OFDM symbol. There are various techniques used for accommodating variations in timing, including the use of the cyclic prefix, training symbols and other techniques. This provides a tolerance for synchronization errors; however the ability of a receiver to accommodate lack of synchronization is limited.
Satellite Broadcasts
FIG. 1 is a diagram depicting the propagation delay effects of a satellite broadcast implemented from a geosynchronous orbit. The satellite itself is 35,786 km above mean sea level; however the distance to any point on the earth is greater according to the distance of that point from the orbital track of the satellite. The propagation delay is represented by arcs 111-119, so that, for example, a receiver near arc 111 would be subject to less delay than a receiver near arc 118. The change in propagation delay is continuous with distance from the satellite's sub-satellite point, so arcs 111-119 are not defined incremental boundaries. This change in propagation distance generally is greatest in the north-south direction, with a declination corresponding to the orbital position of the satellite.
One aspect of shared frequency broadcasting using multiple sources is that the relative delay in receipt of the signals varies according to the position of the receiver with respect to the multiple sources. If the multiple sources are equidistant from the receiver, then signals transmitted at the same time will be synchronized. If the receiver is closer to one transmitter, then the propagation delays will differ. In the case of satellite transmissions, there is a significant propagation delay. In the case of geosynchronous satellites, the radio propagation delay corresponds to 35,786 km above mean sea level plus the skew distance to the receiver established by the geographical latitude of the receiver.
If a signal is to be simultaneously received from both a terrestrial station and a satellite, the transmission of the terrestrial station must be delayed with respect to that of the satellite. This delay changes in accordance with the angle of inclination of the signal, which roughly corresponds to the latitude of the receiver. This delay can be adjusted, and a receiver on the ground can continue to receive both a terrestrial signal and a satellite signal substantially simultaneously, provided that the signal times fall close to being within the time window defined by the cyclic prefix window. It is desirable that the signal times fall within the time window defined by the cyclic prefix window or reasonably close to that time window because this reduces interference. If the signal times fall within the time window defined by the cyclic prefix window, the received signals exhibit a low amount of interference. It is possible to exceed the time window defined by the cyclic prefix by a small amount. Exceeding the time window can result in interference; however, a small amount of interference is deemed acceptable because it does not substantially degrade the received signals.
In the case of terrestrial broadcasts, adjacent stations can have their signals synchronized, or alternatively skewed, in a manner to optimize reception from the multiple terrestrial stations. This is particularly advantageous in regions within the coverage areas where signal strength or signal quality are weakest. The areas covered by satellite broadcasts, on the other hand, generally do not correspond to the terrestrial broadcast areas. As a result, the propagation delay of a satellite transmission when taken at different locations across a given terrestrial broadcast area will vary. The delays in the satellite transmission when taken at different locations across multiple terrestrial broadcast areas will vary to a significantly greater degree, particularly along a generally north-south direction. For this reason, setting synchronization between terrestrial stations in a conventional fashion results in the terrestrial stations being out of sync with the satellite or inoptimally synchronized with the satellite.